How can I import an image sequence into blender
The Frames value in your Image Sequence node is set to 1, so it will only show the first frame of the sequence. Set that value to the total number of frames in that image sequence and you should see it update. You may also want to enable Auto-Refresh so you can see updates as you scrub.

That said, if you're just doing an Alpha-over, it would probably be way more suitable to do this in the VSE.